1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive employing pivot friction compensation.
2. Description of the Prior Art
Disk drives for computer systems typically comprise a disk rotated by a spindle motor with a head actuated over the disk in order to access data recorded in concentric tracks. The head is coupled to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) in order to seek the head to a target track, and then maintain the head over the track (tracking) while performing read or write operations. The position information for servoing the head comes from servo sectors recorded at periodic intervals around the circumference of the tracks. The servo sectors typically comprise coarse position information in the form of a digital track address, and fine position information in the form of servo bursts recorded at precise offsets relative to the centerline of each track.
During the tracking operation, the pivot bearings exhibit a friction (pivot friction) that introduces a non-linear disturbance into the servo system. As the pivot velocity approaches zero, the non-linear effect of the pivot friction dominates the mechanical dynamics of the servo system and degrades tracking performance. The effect of pivot friction is of particular concern in disk drives that exhibit eccentric tracks due, for example, to written-in errors while writing the servo sectors, non-centric alignment of the disk after servo writing (due to the disk slipping), or non-centric alignment of the disk when clamped to the spindle motor shaft (after servo writing with a media writer). In disk drives where the dominant eccentricity is due to a centric offset relative to the spindle motor shaft and the tracks, the velocity of the actuator arm will be sinusoidal with the rotation of the disk while tracking the eccentricity as illustrated in FIG. 1. The resulting pivot friction can be modeled as a square wave having an alternating sign as the pivot velocity approaches zero and reverses direction (mathematically the pivot friction can be represented as fc*sign(v) where v is the pivot velocity).
The prior art has suggested using an accelerometer for estimating the pivot velocity in order to compensate for pivot friction. However, accelerometers are complex and expensive to manufacture which increases the cost and complexity of the disk drive. The use of sophisticated observer based compensation modeling has also been suggested, but modeling error can lead to less than optimal performance.
There is, therefore, a need to compensate for pivot friction in a disk drive without using expensive accelerometers, and to avoid the modeling error of observer based compensation.